In the conditioning of plastic resins for a molding or extruding process, the plastic resins are frequently pelletized and introduced into production via a drying hopper, wherein the plastic pellets are dried for a period of time prior to processing. Generally, plastic resins which are affected by moisture fall into two classifications, namely hygroscopic and non-hygroscopic. Non-hygroscopic resins collect moisture on the surface of the pellet only. This surface moisture can be removed by a relatively simple process involving the application of warm air to the material. Since warmer air has the ability to hold water, when it is passed in a stream over the resin, the moisture tends to leave the surface of the pellet in favor of the warm air stream, and dry resin results. A different situation is present, however, in the case of hygroscopic resins. These resins attract moisture from the ambient air and collect moisture inside the pellet itself. If this moisture is not removed from the pellets prior to processing, harmful splays or defects in the physical properties of the finished product will result. Hence, drying systems must be utilized to deliver dry air as well as heat to the resin pellets, prior to processing, to thereby remove the moisture from the pellets. The drying system for which the present invention is specifically designed is a closed loop system, although use in other type of drying systems is possible. A closed loop drying system generally consists of a desiccant dehumidifier and a drying tank or hopper, wherein the moisture laden air exiting from the hopper is cleaned in a filter, dried in the desiccant dehumidifier, and heated to the desired temperature before being recirculated back to the drying hopper.
Within the drying hopper is a device used to diffuse the clean, dry, warm air such that all of the pellets are contacted during a predetermined period of time. This diffusing device must be constructed in such a way as to allow proper distribution of the air, and at the same time, it should promote the "plug flow" of the material as it passes through the drying hopper. Plug flow is achieved when all of the pellets at any one level move uniformly through the hopper. If the diffuser is not so constructed, funneling or channeling of the material as it flows around the diffuser will occur. As a result of funneling, some of the plastic pellets in the drying hopper will not receive proper exposure to the dry air and others will receive too much, thus reducing the strength and/or appearance of the plastic resin.
Prior art diffusers have typically had either a double conical or a diamond formation. The double conical diffuser is positioned in the hopper base and extends entirely across the diameter thereof. It is comprised of two conical portions, one being smaller than the other and positioned inversely above the larger cone. In this prior art embodiment, the warm, dry air moves in a circular motion within the diffuser and is directed upwards therefrom into the hopper. Due to the air being moved upwards, the materials located in the bottom of the hopper may not be evenly heated and dried upon start-up of the drying system. In turn, this can result in harmful splays and defective products as discussed above. In addition, hot spots may develop at the inlet to the diffuser unless the air is made to enter the diffuser tangentially. The diamond diffuser is also positioned in the hopper base, but it does not extend across the entire diameter. As a result in this instance, some of the material will flow around the diffuser as it travels to the hopper outlet. This creates funneling of the pelletized resin, decreases the uniformity of the heating and drying of the pellets, and hinders the plug flow of the material.
Another disadvantage of both the double conical and diamond diffusers is the difficulty encountered when removal from the hopper base is required, such as for cleaning. Some hoppers in the prior art are provided with a hinged door through which access to the resin pellets may be gained when necessary and through which the diffuser may be removed; however, the door is not always large enough for this to be easily accomplished. Thus, removal of the diffuser becomes an arduous task typically requiring several attempts and various contortions before the diffuser is successfully manipulated through the hopper door.
A strong need therefore exists for a diffuser which evenly distributes the warm, dry air throughout all the contents of the hopper, promotes the plug flow of the resin pellets, and may also be easily removed from the hopper base.